This invention relates to a receiver for spectrum spread communication, and more particularly to a receiver for spectrum spread communication employing a hybrid system.
Conventionally, a spectrum spread (hereinafter referred also to "SS") communication system has been developed and partially put in practice for the purpose of being utilized in various communication fields including office automation, mobile communication, remote control and the like.
The SS communication system satisfactorily exhibits a variety of significant characteristics such as crosstalk-proof characteristics, noise-proof characteristics capable of eliminating interference and the like, because it can transmit a narrow-band signal while spreading it into a broad-band signal. The SS communication system is generally classified into a frequency hopping (FH) system and a direct spread (DS) system.
The FH system is adapted to cause one bit of information to be dispersed into a number of frequencies, so that it causes the circuit construction to be highly complicated although it may exhibit resistance to fading and interference.
The DS system is simplified in circuit construction, however, it is inferior in fading characteristics and the like to the FH system.
In view of the foregoing, a hybrid system in which the FH and DS systems are used in combination with each other to effectively utilize the advantages of both is considered.
FIG. 4 is a block diagram showing a transmitter-receiver device used for a conventional hybrid SS communication system.
In FIG. 4, a base band data signal V.sub.BI which is a transmit data is multiplied by a pseudo-noise (PN) code generated from a PN code generator and then supplied to one of inputs of a mixer 403. The PN code is varied depending upon its applications, its band and the like; however, in general, an M-series code of tens to hundreds of bits is used for this purpose. A frequency synthesizer 401 includes a plurality of signal sources different in frequency and serves to change over output signals of a hopping pattern in response to the PB code form the PB code generator in turn and supplies them to the other input of the mixer 403. The mixer 403 carries out multiplication between the signal form the mixer 404 and that from the frequency synthesizer 401, so that a signal subject to frequency hopping may be transmitted in the form of a radio wave from a transmitting antenna 405.
The above-described transmit signal is received through a receiving antenna 406 and then supplied to one of inputs of a mixer 407. The mixer 407 carries out multiplication between a signal supplied from a mixer 408 and the above described receive signal and then supplies the resultant signal to a demodulator 411. The signal is demodulated in the demodulator 411 and then output in the form of a base-band output signal V.sub.BO. The base-band output signal V.sub.BO corresponds to a base-band input signal V.sub.BI on the transmitter side.
A signal output from the demodulator is supplied to a synchronous circuit 412. The synchronous circuit 412 functions to control the frequency of a PN code output signal generated from the a code generator 410 so as to maximize the output signal of the demodulator 411. The PN code generator 410 is constructed in substantially the same manner as the PN code generator 402 on the transmitter side. The output signal of the PN code generator 410 is supplied to one of inputs of the mixer 408 and a frequency synthesizer 409. The frequency synthesizer 409 is constructed in the same manner as the frequency synthesizer 4101 on the transmitter side and serves to supply, to the other input of the mixer 408, a plurality of frequency signals in the same hopping pattern as on the transmitter side in response to the PN code from the PN code generator 410 in turn. The mixer 408 carries out multiplication between the signals input thereto from the frequency synthesizer 409 and PN code generator 410 and supplies the resultant signal to the other input of the mixer 407. The mixer 407, as described above, mixes the signal form the mixer 408 with the signal from the receiving antenna 406 and then supplies the resultant signal to the demodulator 411.
Synchronism is deemed to be accomplished between the hopping speed on the transmitter side and that on the receiver side when the output signal of the demodulator 411 is maximized by repeating the foregoing operation, resulting in the base-band output signal V.sub.BO corresponding to the base-band input signal V.sub.BI being obtained.
As will be noted from the foregoing, the conventional hybrid SS communication system can transmit a variety of data. However, the conventional system employs a dlay lock loop (DLL) circuit or the like as a synchronism supplementing circuit and a synchronism holding circuit, so that much time is required until synchronism is completed and it is highly difficult to establish complete synchronism.